Viral load and sexual transmission risk

There
is a clear relationship between lower viral load and reduced risk of HIV
transmission.

A high viral load can significantly increase the
risk of sexual transmission.

A low viral load significantly reduces sexual transmision
risk.

A
low viral load when not on treatment may not be as protective as a low
viral load on antiretroviral therapy.

It
is currently unknown whether there is a viral load threshold below which
transmission is not possible.

There is a clear relationship
between lower viral loads and reduced risk of HIV transmission. Whereas a high
viral load can significantly increase the risk of sexual transmission, a low
viral load significantly reduces sexual transmision risk.

The meta-analyses referred to
in these sections include some of the same studies, but examine the impact on
sexual transmission risk in different ways: some according to viral load
regardless of treatment, and some according to treatment’s impact on viral
load.

In 2009, a meta-analysis examining the impact of viral load
on transmission risk (whether or not someone was on treatment) found that a high viral load can significantly increase the
risk of transmission, and that a low viral load significantly reduces the risk.1
The
meta-analysis estimated that out of 1000 HIV-positive individuals with a viral
load below 400 copies/ml regularly
engaging in vaginal sex with an HIV-negative partner, only one transmission
could be expected to occur in the course of a year. In contrast, among 1000
HIV-positive individuals with a viral load above
50,000 copies/ml, at least 90 transmissions could be expected to occur in
the course of a year.

It is currently unknown whether there is a viral load threshold
below which transmission is not possible. The
meta-analysis estimated that the transmission rate for people with a blood
plasma viral load below 400 copies/ml is 1 in 6250. The lowest recorded
threshold of sexual transmission in an individual not on ART included in the
meta-analysis occurred at a viral load of 362 copies/ml.

There may, however, be a difference in the risk of
transmission between people who have low viral loads who are not on treatment,
and people who have low viral load while on antiretroviral therapy. In
untreated people, viral load in blood plasma is less reliably correlated with viral
load in the genital tract, and this may be related to the penetration of antiretrovirals
into seminal and vaginal fluid and rectal secretions.2

Nevertheless, there have since been case reports of
suspected sexual transmission between men even when the blood plasma viral load
was below the limit of detection due to antiretroviral therapy.3

This may be due to the imperfect correlation between viral
load as measured in the blood, and the amount of virus in other body fluids,
including those exchanged during unprotected sex: semen, cervico-vaginal fluids
and/or rectal secretions. This is
covered in the section on Viral load in
semen, cervico-vaginal fluid and rectal secretions.

Related Links

Key studies

A study of heterosexual couples in Uganda, published
in 2000, and is often considered to be the benchmark study confirming that
viral load measured in the blood is the most important factor in determining
whether or not HIV is transmitted following sexual exposure. It found that no
HIV transmission was observed over a 30-month period in the 51 couples where
the HIV-positive partner had consistent viral load measurements in the blood
below 1500 copies/ml.4

In May 2009, Attia and colleagues published a systematic
review and meta-analysis examining all known prospective studies published or
presented between January 1996 and February 2009 on the risk of HIV
transmission through unprotected sexual intercourse according to viral load.1 All were in heterosexual couples.

Of note, the review did not include studies examining the
relationship between viral load and risk of transmission in sex between men,
nor during anal sex, which is also practised by a significant minority of
heterosexual couples, and which is often not reported, particularly in Africa
where the practice is often considered to be taboo.5

Of the ten studies that included HIV-positive individuals not receiving antiretroviral therapy they
calculated that amongst people with a viral load below 400 copies/ml the
transmission rate was 0.16 per 100 person-years (0.0016 or 1 in 6250).

The most recent
analysis of risk of heterosexual transmission according to blood plasma viral
load comes from the Partners in Prevention study, published in 2012.6
This involved 3297 serodiscordant couples in sub-Saharan Africa
who were not on antiretroviral therapy.

There were 151 new
HIV infections during the two-year study (which was originally designed to
assess whether or not the anti-herpes drug, aciclovir, taken by the
HIV-positive partner reduced sexual transmission risk). Of these, 108 originated
from the main partner, as determined by phylogenetic analysis. For this sub-study,
86 linked transmissions with full viral load data from the transmitting partner
were included.

The HIV-positive
partner of each couple had their viral load tested every three to six months
during the study and the HIV-negative partner took an HIV test every three
months. Each time they came to the study centre they were asked about their
sexual behaviour since the previous visit.

Of note, there were
56 transmissions between partners where 100% condom use was claimed (the
majority of couples in the study said they used condoms) and 15 in couples who
claimed to have had no sex since the last visit.

The investigators
found that each tenfold increase in viral load in the transmitting partner
multiplied the risk of infection 2.89-fold.

This can be
summarised as follows:

Viral load in transmitting
partner Per-act
infection risk

1000 copies/ml 0.00028 (One in 3571)

10,000 copies/ml 0.00082 (One in 1220)

100,000 copies/ml 0.0024 (One in 416)

1,000,000 copies/ml 0.0068 (One
in 147)

The Attia review also included new information concerning a
prospective observational study involving 393 monogamous heterosexual couples
in Spain
to determine HIV transmission risks, originally published in 2005. It revealed
that there had been one case of sexual transmission when a HIV-positive
partner, who was not on treatment, had a viral load of 362 copies/ml.7

Two further studies have also reported HIV transmission when
viral load was below 1500 copies/ml (again in untreated individuals): at viral
loads of 600 copies/ml8
and 1497 copies/ml.9

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap

close

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends
checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member
of your healthcare team for advice tailored to your situation.